1. Field of the Invention
The present invention relates in general to ultrasonic transducer units used for depth finding, fish finding and the like on marine pleasure craft, and in particular to self-aligning transducer apparatuses for marine applications which use gravity to align an electroacoustic transducer in a desired direction.
Electronic depth finders employing sonar transducers have been known and used for many years. They are also called by various other names, such as depth flasher, fish finder, depth meter, depth recorder and video sonar. The word sonar is an acronym for "sounding, navigation and ranging." A sound tone is created electronically, typically using a piezoelectric crystal transducer, and sent out into the water in a cone-like beam. The speed of sound in water is about 4,950 feet per second. When that sound wave hits a solid object, part of the sound is reflected back to the source as an echo, and is picked up by the same transducer. By carefully timing how long it takes for an echo to return, it is possible to determine the distance between the source and the object.
The relatively low-cost electronic sonar systems that are widely used today by fishermen and pleasure boaters employing a main control box located in the boat and having an electronic device that creates an electrical signal which travels from the main box through an electrical cable to a transducer device that is immersed in the water. The typical electroacoustic transducer operates at a given ultrasonic frequency in a range between about 25 KHz and 250 KHz, and is a controlled sound wave energy transmitter and receiver that converts the electrical impulse into a high frequency sound impulse that is inaudible to fish as well as to humans. The transducer is typically aimed at the bottom of the body of water or at objects beneath the pleasure craft. The echo bounces back to the transducer which reconverts that sound back into an electrical impulse which travels back up the cable and, after appropriate signal processing, shows up on a readout of the main control box as a blip or mark that normally can be read directly in terms of feet or fathoms.
One of the chief problems experienced by pleasure boaters or fishermen, especially on rough bodies of water, is that the direction in which the cone of the transducer points is not stable. In conventional pleasure boats, the transducer is normally rigidly attached to the hull or to a support bracket off the transom. Thus as the boat pitches or rolls on account of wave action, on account of the boat keeling to one side in a turn, or on account of changes in speed, the direction of the cone emanating from the transducer varies accordingly. This results in random unstable and changing echo return signals. This random ranging produces a chart or display filled with undesirable clutter and fails to clearly define the desired field of view or its contents.
A number of sonar systems now employ recording mechanisms which produce a paper chart showing the bottom being scanned by the transducer, or display the same kind of information on a display screen such as a cathode ray tube (CRT). So, as the direction of the cone changes, the accuracy and usefulness of the resulting graphical display is seriously impaired. When the water is rough, the display may go blank if the return signals are too scrambled to decode, or the sonar system may fail to show fish that are in fact in the the water below the boat, even though the same system would show clearly such fish in calmer waters when the boat was more stable. Thus it would be highly desirable to provide a relatively inexpensive and reliable transducer apparatus that provides the stability needed in a transducer in order to have a stable directional beam emanating therefrom.
A number of systems have been developed over the years in an effort to provide a stabilized transducer which is capable of sending out a sonar signal in a stable direction in spite of the rolling and/or pitching of the craft. All of these devices known to me are somewhat bulky and complex in construction. A number of these devices employ the force of gravity in an effort to provide a stabilized transducer. Such systems are disclosed in the following U.S. patents:
______________________________________ No. U.S. Pat. No. Granted Inventor ______________________________________ 1. 2,019,497 1935 Kuntze 2. 2,407,697 1946 Williams 3. 2,759,783 1956 Ross 4. 2,832,944 1958 Kessler 5. 3,518,676 1970 Kirknes 6. 3,553,638 1971 Sublett 7. 3,563,335 1971 Holmes 8. 4,144,518 1979 Minohara et. al. ______________________________________
The devices disclosed in the first, second, fourth and sixth listed patents employ a pendulous mountings for a transducer assembly. Further, a number of these patents disclose transducers which are mounted in liquid filled chambers. However, as disclosed in the fifth patent, namely U.S. Pat. No. 3,518,676, these prior art systems suffered from a damping influence of the liquid which at times impedes the swinging movement of the pendulously mounted transducer to the extent that it would not to be able to compensate for the pitching and rolling movements of the ship and the sea. Thus the system disclosed in the fifth patent provides a servo control system to take care of this problem. The seventh patent discloses a mechanically stabilized platform which is strictly mechanical in operation and is designed to overcome the limitations of the prior art purely mechanical systems and to obviate the need to employ servo control systems for applying corrective position adjustments to a transducer. The eighth patent is also mechanical in its approach. However, both of the seventh and eighth patents disclose systems which are fairly complex and cannot easily be put into the small spaces typically occupied by conventional transducers on pleasure craft.
Thus, it is apparent that there is still a significant need for a small, relatively low-cost, reliable self-aligning transducer apparatus which does not require sophisticated electronic servo controls for stabilization. Further, there is a need for a small self-contained and self-aligning apparatus which in spite of being immersed in a liquid is not so heavily damped by the liquid as to be unable to keep up with the pitching or rolling of the boat. Of course, the small vessels, such as pleasure craft, are fairly lightweight and have a smaller displacement, and therefore roll and pitch back and forth much more actively than do larger commercial vessels. Hence, any self-aligning transducer for pleasure craft must be able to respond even more quickly and faithfully than the prior art systems for larger commercial vessels disclosed in the foregoing patents.
In light of the foregoing problems and needs, it is an object of the present invention to provide a self-aligning transducer apparatus including an electroacoustic transducer which continuously points in a predetermined direction independent of the pitching and rolling of a marine craft to which it is attached.
It is another object of the present invention to provide a compact, low-cost, highly reliable self-aligning transducer apparatus. It is a further object of the present invention to provide a self-aligning transducer apparatus with the ability to respond quickly and faithfully to the pitching and rolling of even small marine pleasure craft, such as sport boats and bass boats used for fishing.